1. Field of the Invention
This invention resides in the field of infrared-transparent materials, and also in methods and materials for improving the durability and transmissivity of structural materials.
2. Description of the Prior Art
Infrared sensors are used in various kinds of aircraft, both military and civilian, for a variety of reasons. Heat-seeking missiles, for example, use infrared sensors can guide themselves to a target aircraft by sensing the hot exhaust from the aircraft. Likewise, military targets on a battlefield can be observed with infrared sensors, and in general any object can be detected or its temperature measured by the infrared radiation emitted by the object.
Since infrared sensors are usually fragile in construction and susceptible to damage from external objects or harsh environments, the sensors are typically placed behind protective windows. Many of these windows are curved or dome-shaped, such as those protecting sensors that are placed in the noses of infrared-guided missiles and those protecting sensors that reside in a pod or turret mounted on the exterior of a helicopter. In both cases, the curved shape of the window reduces the aerodynamic drag on the aircraft. Curved or flat, however, the window must be highly transmissive of infrared radiation, highly resistant to external forces and conditions such as wind, rain, radiation, thermal shock, and chemical attack, and sufficiently strong to withstand aerodynamic forces and particle impacts. A material commonly used in infrared windows is sapphire, a form of aluminum oxide, which is favored because of its high transmissivity of infrared radiation and its high resistance to adverse environmental conditions. As greater demands are made on aircraft and weaponry, however, window materials that can withstand even higher heating rates, greater mechanical loads, and harsher environments are needed. Sapphire is also expensive and difficult to machine into precise curvatures or other shapes. Furthermore, when large windows are needed, the need for structural strength requires them to be of greater thickness, and the transmissivity of sapphire decreases with increasing thickness. Other materials, such as zinc sulfide (ZnS), zinc selenide (ZnSe), germanium (Ge), and gallium arsenide (GaAs), retain their transmittance as their thickness increases, but they lack the necessary resistance to external damage due to rain and particle impact.
One class of alternative materials that has been investigated for its low cost and high performance is polycrystalline materials. These materials have their limitations as well, however, notably a high degree of optical scatter and inferior mechanical strength. Optical scatter reduces the transmissivity of the material, and the inferior mechanical strength is generally accompanied by poor resistances to thermal shock, rain, and particle or projectile impacts.
All citations appearing in this specification, including published papers, patents and Internet websites, are hereby incorporated herein by reference in their entirety for all purposes legally capable of being served thereby.